The invention relates to pressure swing adsorption processes and more particularly to PSA processes for the production of high purity oxygen (e.g. oxygen having a purity of 90-95 vol. % O2). More particularly, the invention is directed towards increasing the adsorbent productivity while reducing the O2 product cost.
There has been significant development of the various PSA, VSA and VPSA methods for air separation over the past thirty years, with major advances occurring during the last decade. Commercialization of these processes and continued extension of the production range can be attributed primarily to improvements in the adsorbents and process cycles, with advances in adsorber design contributing to a lesser degree. Highly exchanged lithium molecular sieve adsorbents, as illustrated by Chao in U.S. Pat. No. 4,859,217, are representative of advanced adsorbents for oxygen production. Advanced adsorbents of the types mentioned above are the result of improvements in equilibrium properties.
Improving process efficiency and reducing the cost of the light component product can be accomplished by decreasing the amount of adsorbent required and by increasing the product recovery. The former is generally expressed in terms of bed size factor (BSF) in lbs adsorbent/TPDO (ton per day of contained O2), while the latter is simply the fraction of light component (i.e. oxygen) in the feed (i.e. air) that is captured as product. Improvement in adsorbents and reduction in cycle time are two primary methods of reducing BSF.
While shorter cycles lead to shorter beds and higher adsorbent utilization, product recovery generally suffers unless adsorption rate is increased. This phenomena can be ideally characterized in terms of the size of the mass transfer zone (MTZ), i.e. the mass transfer zone becomes an increasing fraction of the adsorbent bed as the bed depth decreases. Since the adsorbent utilization with respect to the heavy component (e.g. nitrogen) is much lower in the MTZ than in the equilibrium zone, working capacity declines as this fraction increases.
The effect of particle size upon the size of the MTZ is conceptually straightforward in a single long adsorption step where a contaminant in relatively low concentration is removed from the feed stream on the basis of its higher equilibrium affinity to the adsorbent. When the adsorbate/adsorbent combination is characterized by a favorable isotherm, a steady state transfer zone is envisioned that moves through the adsorber at a constant speed. Distinct equilibrium and mass transfer zones can be identified in the process. Under such conditions, and when the resistance to mass transfer is dominated by intraparticle pore diffusion, it has long been recognized that reducing the adsorbent particle size results in higher adsorption rates and smaller mass transfer zones. Unfortunately, pressure drop across the adsorbent bed increases with decreasing particle size and leads to difficulty in particle retention in the bed and an increased tendency for fluidization.
This ideal concept becomes blurred when the isotherms are unfavorable and/or the mass transfer zone is continuously developing or spreading throughout the adsorption step. Adding the remaining minimum steps of depressurization, desorption and pressurization to create a complete adsorption process cycle further complicates the behavior and character of the mass transfer zone. Nevertheless, the idealized concept of the MTZ has been applied in the prior art as a basis to affect improvements in process performance.
Jain (U.S. Pat. No. 5,232,474) discloses improving adsorbent utilization by decreasing the adsorbent volume and/or increasing the product purity, wherein the removal of H2O and CO2 prior to cryogenic air separation is described using a pressure swing adsorption (PSA) process. The adsorber is configured entirely with alumina or with layers of alumina and 13X molecular sieve adsorbents. Smaller particles (0.4 mm to 1.8 mm) are used to achieve a smaller bed volume.
Umekawa (JP Appl. No. 59004415) shows a lower pressure drop and smaller adsorber for air purification by using a deep layer of large particles (3.2 mm) followed by a shallow layer of small particles (1.6 mm) of the same adsorbent. The bed size and pressure drop of this layered configuration are lower than for beds constructed either of all 3.2 mm or all 1.6 mm particles. The 1.6 mm particles occupy only a small fraction (low concentration part) of the mass transfer zone in the layered configuration.
Miller (U.S. Pat. No. 4,964,888) has suggested using larger particles ( greater than 14 mesh or 1.41 mm) in the equilibrium zone and small particles ( less than 14 mesh) in the mass transfer zone. This reduces the size of the MTZ while minimizing the excessive pressure drop increase that would occur if only small particles were used in both zones. Cyclic adsorption process times greater than 30 s are indicated.
Garrett (UK Pat. Appl. GB 2 300 577) discloses an adsorption apparatus containing particles in the size range between 6 mesh (3.36 mm) and 12 mesh (1.68 mm) deployed in either discrete layers or as a gradient of sizes with the largest particles located near the feed inlet and the smallest particles located downstream near the outlet of the adsorber in both configurations.
Very small adsorbent particles (0.1 mm to 0.8 mm) are necessary for the fast cycles and high specific pressure drop that characterize a special class of processes known as rapid pressure swing adsorption (RPSA). Typical RPSA processes have very short feed steps (often less than 1.0 s) operating at high feed velocities, include a flow suspension step following the feed step and generally have total cycle times less than 20 s (often less than 10 s). The behavior of the adsorption step is far removed from the idealized MTZ concept described above. In fact, the working portion of the bed is primarily a mass transfer zone with only a relatively small equilibrium zone (in equilibrium with the feed conditions) in RPSA. A major portion of the adsorber is in equilibrium with the product and provides the function of product storage. The high pressure drop (on the order of 12 psi/ft)/short cycle combination is necessary to establish an optimum permeability and internal purging of the bed which operates continuously to generate product.
RPSA air separation processes using 5A molecular sieve have been described by Jones et al. (U.S. Pat. No. 4,194,892) for single beds and by Earls et al. (U.S. Pat. No. 4,194,891) for multiple beds. Jones has also suggested RPSA for C2H4/N2, H2/CH4, H2/CO and H2/CO/CO2/CH4 separations using a variety of adsorbents. The RPSA system is generally simpler mechanically than conventional PSA systems, but conventional PSA processes typically have lower power, better bed utilization and higher product recovery.
In somewhat of a departure from the original RPSA processes, Sircar (U.S. Pat. No. 5,071,449) discloses a process associated with a segmented configuration of adsorbent layers contained in a single cylindrical vessel. One or more pairs of adsorbent layers are arranged such that the product ends of each layer in a given pair face each other. The two separate layers of the pair operate out of phase with each other in the cycle. The intent is for a portion of the product from one layer to purge the opposing layerxe2x80x94the purge fraction controlled by either a physical constriction placed between the layers and/or by the total pressure drop across a layer (ranging from 200 psig to 3 psig). Particles in the size range of 0.2 mm to 1.0 mm, total cycle times of 6 s to 60 s, adsorbent layer depths of 6 inches to 48 inches and feed flow rates of one to 100 lbmoles/hr/ft2 are broadly specified. An optional bimodal particle size distribution is suggested to reduce interparticle void volume. The process is claimed to be applicable to air separation, drying, and H2/CH4, H2/CO and H2/CO/CO2/CH4 separations.
Alpay et al. (Chem. Eng. Sci., 1994) studied the effects of feed pressure, cycle time, feed step time/cycle time ratio and product delivery rate in RPSA air separation for several ranges of particle sizes (0.15 mm to 0.71 mm) of 5A molecular sieve. His study showed that process performance was limited when adsorbent particles were either too small or too large. This was because ineffective pressure swing, low permeability and high mass transfer resistance (due to axial dispersion) were limiting at the lower end of particle size range, while high mass transfer resistance became limiting due to the size of the particles at the larger end of the particle size spectrum. Alpay found maximum separation effectiveness (maximum O2 purity and adsorbent productivity) for particles in the size range of 0.2 mm to 0.4 mm.
RPSA is clearly a special and distinct class of adsorption processes. The most distinguishing features of RPSA compared to conventional PSA can be described with respect to air separation for O2 production. The pressure drop per unit bed length is an order of magnitude or more larger and the particle diameter of the adsorbent is usually less than 0.5 mm in RPSA. Total cycle times are typically shorter and the process steps are different in RPSA. Of these contrasting features, pressure drop and particle size constitute the major differences.
Other patents suggest the use of small particles in conventional PSA processes. Armond et al. (UK Pat. Appl. GB 2 091 121) discloses a superatmospheric PSA process for air separation in which short cycles (xe2x89xa645 s) are combined with small particles (0.4 mm to 3.0 mm) to reduce the process power and the size of the adsorbent beds. Oxygen of 90% purity is produced under the preferred cycle times of 15 s to 30 s and particle sizes of 0.5 mm to 1.2 mm.
Hirooka et al. (U.S. Pat. No. 5,122,164) describes 6, 8 and 10-step VPSA processes for separating air to produce O2. While the main thrust of this patent is the cycle configuration and detailed operation of the various cycle steps to improve yield and productivity, Hirooka et al. utilize small particles to achieve faster cycles. A broad particle range is specified (8xc3x9735 US mesh or 0.5 mm to 2.38 mm), but 12xc3x9720 US mesh or 0.8 mm to 1.7 mm is preferred. Half-cycle times of 25 s to 30 s are indicated (total cycle times of 50 s to 60 s).
Hay et al. (U.S. Pat. No. 5,176,721) also disclose smaller particles to produce shorter cycles, preferably in air separation. A vertical vessel with horizontal flow across the adsorbent is depicted. Broad range characteristics include particles less than 1.7 mm diameter, cycle times between 20 s-60 s and pressure drop across the adsorbent less than 200 mb (2.85 psig).
An alternative configuration includes an upstream portion of the adsorbent bed with particles of size greater than 1.7 mm, in which case the particle fraction smaller than 1.7 mm comprises 30% to 70% of the total adsorbent mass. The aspect ratio of the bed (largest frontal length to bed depth ratio) is specified to be between 1.5 and 3.0. Small particle fraction alternatives of 0.8 mm to 1.5 mm and 0.4 mm to 1.7 mm are also given, as well as adsorbent pressure drop as low as 50 mbar (0.7 psig).
Wankat (CRC Press, 1986; Ind. Eng. Chem. Res., 1987) describes a concept that he terms xe2x80x9cintensificationxe2x80x9d whereby decreased particle diameter is employed to produce shorter columns and faster cycles. By non-dimensionalizing the governing mass balance equations for the adsorption process, a set of scaling rules are suggested which preserve the performance of the process in terms of product recovery, purity and pressure drop while increasing the adsorbent productivity. These theoretical results are based upon the similarity of dynamic adsorption behavior (at the same dimensionless times and column locations). The similarity concept presumes an idealized constant pattern MTZ, with the length of the mass transfer zone (LMTZ) directly proportional to the square of the particle diameter when pore diffusion is controlling. Furthermore, decreasing LMTZ increases the fraction of bed utilized. Wankat indicates that increasing (L/LMTZ) beyond a value of two to three, where L is the bed depth, results in minimal improvement in the fractional bed utilization. A layer of small-size particles placed on top of a layer of large-size particles is also suggested as a way to sharpen the mass transfer front. Some of the practical limitations to smaller scale and faster operation have been noted and include fluidization, column end effects, wall channeling and particle size distribution. The intensification concept was later extended to include non-isothermal and non-linear equilibrium effects in PSA processes by Rota and Wankat (AIChE J., 1990).
Moreau et al. (U.S. Pat. No. 5,672,195) has suggested higher porosity in zeolites to achieve improved O2 yield and throughput in PSA air separation. A preferred porosity range of 0.38 to 0.60 is claimed in conjunction with a minimum rate coefficient. Moreau et al. state that commercially available zeolites are not suitable for their invention since porosity is lower than 0.36.
Lu et al.(Sep. Sci. Technol. 27, 1857-1874 (1992); Ind. Eng. Chem. Res. 32: 2740-2751 (1993)) have investigated the effects of intraparticle forced convection upon pressurization and blowdown steps in PSA processes. Intraparticle forced convection augments macropore diffusion in large-pore adsorbents where the local pressure drop across the particle is high and where the pores extend completely through the particle. The higher intraparticle permeability is associated with high particle porosity, e.g. porosities (xcex5p)=0.7 and 0.595.
It is therefore an object of the invention to increase efficiency, reduce cost and extend the production range of high performance adsorption processes for the separation of gases.
It is a further object of the invention to increase efficiency, reduce cost and extend the production range of high performance adsorption processes for production of oxygen.
The invention is based upon the recognition that intrinsic sorption rates, in particular the effective macropore diffusivity, of adsorbent materials have a significant impact upon process performance. In a preferred embodiment an adsorption process uses an adsorbent zone comprising an adsorbent selected from the group consisting of A-zeolite, Y-zeolite, NaX, mixed cation X-zeolite, LiX having a SiO2/Al2O3 ratio of less than 2.30, chabazite, mordenite, clinoptilolite, silica-alumina, alumina, silica, titanium silicates and mixtures thereof, wherein said adsorbent has a mass transfer coefficient for nitrogen of kN2xe2x89xa712 s31 and an intrinsic diffusivity for N2, when measured at 1.5 bar and 300K, of Dpxe2x89xa73.5xc3x9710xe2x88x926m2/s. Other preferred embodiments include the development of process parameters around which such materials should be used and preferred methods for increasing effective diffusivity.